西赤道太平洋所羅門海ODP1115B站位上部上新統至更新統鈣質超微化石生物地層硏究

Abstract

西太平洋暖池 (Western Pacific Warm Pool, WPWP) 為全球重要的熱量及水氣來源，近年來探討西太平洋暖池的研究日益增多，本研究利用海洋鑽探計畫 (Ocean Drilling Program) 第180航次於西太平洋暖池南緣的所羅門海域所鑽取的ODP 1115B海洋岩芯(9 o 11.382’S, 151 o 34.437’E, 水深1149公尺)，檢視深度0至216.43公尺的鈣質超微化石，自種屬組合來討論晚上新世迄今的生物事件，輔以重新判讀的磁地層，與前人研究的氧同位素地層做交叉對比，使其年代模式更加精確，以提供後續研究之用。再者鈣質超微化石的豐度分佈，可視為良好的氣候參數代用指標，對尚缺乏詳細鈣質超微化石研究的西赤道太平洋而言，不啻能成為該地區第一個高解析度鈣質超微化石生物地層研究的典範。 在ODP 1115B 216.43公尺的岩芯樣品中總計可定出十四個鈣質超微化石生物事件。其中FO (First Occurrence) Emiliania huxleyi、LO (Last Occurrence) Pseudoemiliania lacunosa、LO Reticulofenestra asanoi、LO large Gephyrocapsa、FO large Gephyrocapsa、LO Calcidiscus macintyrei、LO Discoaster brouweri等七個生物事件具全球等時性。非等時性的生物事件中，reen (reentrance) medium Gephyrocapsa因該種屬對暖水溫的偏好而與緯度分佈具高度相關；FO medium Gephyrocapsa在西太平洋相近岩芯發揮良好對比特性；而FO Reticulofenestra asanoi、LO Helicosphaera sellii、LO Discoaster pentaradiatus、LO Discoaster surculus and LO Discoaster tamalis等五個生物事件則呈現不規律的異時性。 鈣質超微化石豐度亦可作為氣候參數代用指標，首先暖水種Discoaster屬的豐度變化可作為海水溫度的代用指標，指示自三百萬年前巴拿馬地峽關閉之後，西赤道太平洋地區受北半球冰蓋發育影響的過程。而Florisphaera profunda的豐度變化可指出三百萬年來此地區赤道表面海流與湧升流的強弱消長。Reticulofenestra asanoi的絕滅和暖水種Gephyrocapsa omega豐度的急劇減少則可呼應九十萬年前的中更新世事件，此事件導因於第四紀冰期與間冰期的旋回週期從四萬年的地軸傾斜角轉變為十萬年的繞日偏心率變化，且振幅加劇，可能由於全球冰川體積增加，使得適應暖水之海洋浮游生物絕滅。

The Western Pacific Warm Pool (WPWP) is the largest source of heat and moisture on the Earth surface. For its importance the paleoceanographic studies of the WPWP have increased dramatically during the past decade. In an attempt to improve the resolution and accuracy of chronology of the stratigraphic records in the WPWP for the last 3 million years, the current study establishes calcareous nannofossil biostratigraphy of ODP site 1115B (9o11.382’S, 151 o 34.437’E, water depth 1149 m) in the Solomon Sea with a newly modified magnetostratigraphy of the Upper Pliocene-Pleistocene. The existing, unpublished oxygen isotope stratigraphy (Ke, 2008) provides a stratigraphic framework in which the chronological significances of the 14 recognized bioevents were evaluated against published results of other sites. Semi-quantitative estimations of relative abundances of environmentally-sensitive nannofossil taxa are used to infer paleoceanographic changes. The nannofossils in samples of the upper 216.43 mbsf of ODP1115B were identified under a light microscope at 1250X magnification augmented with additional scanning electronic microscopy when necessary. Among the 14 recognized bioevents, seven are of good correlation markers because of their global synchroniety. They are the first occurrence (FO) of Emiliania huxleyi, the last occurrence (LO) of Pseudoemiliania lacunosa, LO Reticulofenestra asanoi, LO large Gephyrocapsa, FO large Gephyrocapsa, LO Calcidiscus macintyrei and LO Discoaster brouweri. Seven of the bioevents are considered to be diachronous, including the reentrance (reen) of medium Gephyrocapsa, FO Reticulofenestra asanoi, LO Helicosphaera sellii, FO medium Gephyrocapsa, LO Discoaster pentaradiatus, LO D. surculus and LO D. tamalis. The reason of reen medium Gephyrocapsa being diachronous is that the medium-sized Gephyrocapsa omega (= G. parallela) favored warm water conditions and thereby shows a latitudinal heterogeneity of their first occurrence during marine oxygen isotope stages (MIS) 25-29. Changes in nannofossil assemblage composition signify several major climatic changes during the past 3 million years. The abundance of Discoaster spp. is used as an indicator of warm conditions. The major drops of Discoaster abundance during 2.8-2.6 Ma and 2.5-2.4 Ma are indicative of decrease of sea-surface temperature as resulted from the onset of northern Hemisphere glaciation and its subsequent major ice-sheet expansion. The relative abundance of Florisphaera profunda may indicate the strength of upwelling and therefore change in thermocline depth. The extinction of Reticulofenestra asanoi and the dramatic reduction of Gephyrocapsa omega at about 0.9 Ma related to the mid-Pleistocene revolution when the dominant periodicity of global climatic variation changes from 41 ka to 100 ka, as well as a major step-wise increase of global ice volume took place.